1. Field of the Invention
The present invention relates to a thin film forming apparatus and a method of cleaning the same. More specifically, the present invention relates to techniques for removing deposits deposited on the surfaces of component members of a thin film forming apparatus during a process for forming a thin film on an object to be processed, such as a semiconductor wafer.
2. Description of the Related Art
Thin films, such as a silicon dioxide film and a silicon nitride film, are formed on an object to be processed, such as a semiconductor wafer, by the following CVD (Chemical Vapor Deposition) process to fabricate semiconductor devices.
A wafer boat holding a plurality of semiconductor wafers is loaded into a reaction tube of a thermal processing apparatus heated at a predetermined loading temperature by a heater. Then, the interior of the reaction tube is heated at a predetermined process temperature by a heater, and gases in the reaction tube are discharged through the exhaust port thereof for evacuation to reduce pressure in the reaction tube to a predetermined pressure. After the interior of the reaction tube has been stabilized at a predetermined temperature and a predetermined pressure, film forming gases are supplied through a process gas supply pipe into the reaction tube. The film forming gases undergo thermal reaction in the reaction tube, and a reaction product deposits on surfaces of the semiconductor wafers to form thin films on the surfaces of the semiconductor wafers.
The reaction product produced during thin film formation deposits not only on the surfaces of the semiconductor wafers but also on the inner surface of the reaction tube and surfaces of members exposed to an atmosphere in the reaction tube, such as jigs placed in the reaction tube. The amount of the deposited reaction product increases and, eventually, the deposited reaction product comes off the surfaces and produces particles. Particles that adhere to the semiconductor wafers reduce the yield of semiconductor devices fabricated on the semiconductor wafers.
To prevent such a problem, a cleaning gas, such as a mixed gas of fluorine and a halogen-containing acid gas, is supplied into the reaction tube heated at a predetermined temperature by the heater to remove the reaction products deposited on the inner surface of the reaction tube by dry etching (See JP 3-293726 A, for example.).
The dry etching of the reaction product with the cleaning gas supplied into the reaction tube damages a material forming the reaction tube, such as quartz, and many minute cracks are formed in the surface of the reaction tube. Such minute cracks roughen the inner surface of the reaction tube. Consequently, the surface area of the inner surface of the reaction tube increases, which causes a reduction in the deposition rate.
The reaction product can be removed and the surface area of the reaction tube can be decreased (i.e., planarization) by cleaning the wall surface of the reaction tube by wet cleaning (wet etching) using a hydrofluoric (HF) acid solution. However, wet etching requires troublesome manual cleaning, and disassembling, reassembling and adjusting the thermal processing apparatus. Consequently, the thermal processing apparatus is kept inoperative for a long time, in other words, a long downtime of the processing apparatus occurs, resulting in reduction in the rate of operation of the thermal processing apparatus.